Transmitter modulation



Nov. 1, 1938. P11-5CH 2,135,198

TRANSMITTER MODULATION Y Filed April 17. 1936 -2 Sheets-Sheet l l INVENTOR HELM/7' P/ 756// WML,

ATTORNEY y Nov. 1, 1938.. H P11-5CH i 2,135,198

TRANSMITTER MODULATION Filed April 17. 1936 2 Sheets-Sheet 2 ATTORNEY pagina Nov. 1', 193s TRANSMITTER MODULATION Helmut Pitsch, Berlin-Steglitz, Germany, assignor to Radio Corporation of America, a corporation of Delaware Application April 17, 1936, Serial No. 74,838 In Germany December 10, 1934 3 Claims.

` My present invention relates to a method of and means for improving the economy o-f operation of transmitters.

'Ihe general tendency in modern construction of transmitters is to use ever greater powers in the last stage. Where such large powers are ooncerned as are sent out by large modern broad-` cast stations, economy of operation plays an important part. For this reason, the so-called class A amplification wherein the working point of the tube is located in the middle of the useful portion of the tube characteristic curve,is used less seldom than class B amplification wherein a working point at the lower bend is used or class C amplification wherein a working point below the lower knee is used.

'Ihe invention oiers an advantage in all of these three classes of operation. l In the case of class A amplification, the operating point may be chosen ata point lower than normally usedso that the efficiency of operation is increased. This, to be-sure, would be possible fundamentally speaking also in the use of class B amplification,`A

although it might be that class B amplication is not to be employed for some reason or another. When the basic idea of this invention is used for class B and C amplification, the eiiiciency of the various tubes with reference to the power or` per-1 formance is not increased, though there is an economy in amplier apparatus.

The invention consists in that, in addition.' to the modulated radio frequency potential; the Y modulation potential is impressed upon the grid of the tubes in such `a phase that, with growing radio frequency amplitude, the working point upon the tube characteristic is shifted by the superposed modulation frequency in the direction of larger plate current, and that at the same .time the operating point in class A amplification is placed lower than is normally the case, and in class B and C amplification higher than normally. In describing my invention in detail reference will be made to the attached Vdrawings wherein the curves of Figure 1 illustrate a tube operated 4.5 in accordance with my invention and is to be compared with class A operation as illustrated in Figure 2. Figure 3 illustrates further tube operation in accordance with the present invention. Figures 4 and 5 illustrate application of my in- 50 vention to class B or C operation; while Figures 6, 7, and 8 illustrate the essential elements of circuit arrangements for carrying out my invention. Referring to the attached drawings, Figure 2 represents the well known class A amplification 55 principle. Complete or 100 percent modulation (ol. 25o- 17) shall be assumed for the sake of illustration. However, the circumstance that this mode of amplication is not economical can be seen even from this fact that during the periods when the radio frequency potential is of Zero value, the 5 efliciency is also zero because the plate direct current preserves its full value.

Now, this disadvantage is obviated by the present invention. In the case of Figure 1 the operating point A has been so chosen and set that it 10 is spaced apartfrom the bottom bend a distance which is equal to one-fourth the portion of the characteristic which is utilized. a denotes the modulated radio frequency which is to be amplified, while b denotes the modulation frequency 1 which is superposed according to this invention. The resultant grid voltage has then a shape as shown by c. It will be noted that the radio frequency varies about the audio frequency voltage which is indicated by dotted lines. The plate current whichrresults from this mode of operation with this voltageris shown on the right hand side in Figure 1. The mean audio frequency vvalue also here is shown by dotted lines. The mean value of thisaudio frequency component is 25 the direct current m which is half as large as the direct current m which corresponds to the operating point under conditions of normal class A amplification. As a result the efficiency of operation has been raised. 30 Y The audio frequencyl component when being fed, say, to the sending antenna drops out so that the radio frequency current which is sent out has the same appearance as'the voltage a that has been fed in, Figure 1; hence, nothingV is 35 changed compared with the normal amplification. It is to be noted that in rectilinear ampliiication the plate current has the same appear-Y ance as the grid voltage c so that the plate current may again be imagined to be divided into 40 the components a and b. y

The method `heredisclosed should not be confused with a method known in the earlier art and which consists in deriving from the modulation frequencies a direct current whose magnitude corresponds to the modulation amplitudes, and in shifting by a characteristic direct current voltage the` tube workingY point in such a way that in spite of diierent modulation amplitudes, there will always be preserved the same modulation percentage. Contradistinct from this known method, the one here disclosed offers the advantage that the shaded oscillations have the identical appearance as in normal amplification. The unmodulated carrier-wave component,

therefore, has a constant amplitude so that automatic Volume control at the receiving end is possible as previously in the same measure, though in known methods this would be accomplishable only by the aid of special auxiliary frequencies.

In order to demonstrate that the said known method described `in the preceding paragraph, according to another object of this invention, can be used simultaneously with the method here disclosed, if modification of the working point and consequent change of the output relative to the controlling wave' is acceptable, it is shown in Figure 3 what the oscillations'look like when the modulation amplitude is half as large as in Figure 1. It will be noted that the working point may be shifted along the characteristic, according to the known method before referred to, by the derived direct current voltage further in downward direction without the envelope experiencying any change. By the simultaneous use of both methods, e, g., the known method modified in accordance with my disclosure, as will thus be seen, the efficiency is still further increasable.

The fact that the method may be used also in connection with class B and C amplification has been previously pointed out. Figure 4 shows, by way of example, class C type amplification and the same in which the invention is applied according to Figure 5. In Figure 4 the working point has been shifted to the lefthand side to substantially cut oif carrier output in the absence of modulation, in order that the modulation percentage may be raised. The plate current is shown on the righthand side on top. After elimination of the audio frequency component the oscillations lock as indicated below the horizontal line in Figure 4. It will be noted that'by the severance of the negative radio frequency amplitudes, the amplitude of the envelope has beenY diminished one-half, and this must be compensated by more amplification. The same is true for class B operation.

By superposition of the modulation frequency according to this invention, as shown in Figure 5, nothing is lost of the modulation amplitude. In other words, a gain in amplification is insured in comparison with Figure 4.

Since obviously the highest power appears in the last stage of a transmitter, the invention applied to this stage offers the greatest advantage. Thus, it is possible to utilize the invention in the last stage only. It may, moreover, also be applied to several stages, but only to those that follow the modulation stage, since it is only in these stages that modulated high frequency is found.

For the sake of simplification, it is assumed in Figures 6, 7, and 8 that thelast stage follows the modulation stage.

According to Figure 6, the high frequency wave is conducted across the transformer T, to the modulation stage M operating, for instance, with grid voltage modulation. The tone frequency potential is applied across transformer T1 to the grid circuit of tube M. The modulated high frequency is passed across transformer T2 to the stage L and from there tothe antenna A. The

audio frequency to be superposed in accordance with the invention, is amplified in the tube N and supplied to the grid circuit of tube L. While the tube Mfor the purpose of modulation,` is negatively biased to such 'a high degree that it operates 1in Vthe lower bend, the tube L is biased negatively tosuch an extent only as was shown with reference to Figure 1. In the case of class B and C amplification, the biasing potential for the tube L is chosen in the manner described with reference to Figure 5. Hence, such biasing potential will be chosen that the limit curve of the high frequency fully controls the lowest region of the straight part of the characteristic. As is seen from Figure 1, the audio frequency potential superposed on the tube L must, if the invention is to be fully exploited, be at least approximately as high as the limit curve of the modulated high frequency. If it should be lower, the negative high frequency potentials would not be sufciently Vdisplaced towards the right, but would be partially suppressed, since they would then be at the left ofthe characteristic, a condition which may lead to non-linear distortions. To admit a certain tolerance towards higher or lower value, the working point A will preferably be placed slightly higher. In this case, a deviation of the low frequency amplitude does not immediately cause distortions.

In practice, the characteristic has no sharp bend at the bottom, but reveals a greater or lesser curvature so that, therefore, the working point must be placed at a correspondingly higher point.

It will be understood that care must be taken that through proper polarity of the low frequency transformers in the input or output circuit of tube N, the low frequency of the tube L will be supplied in the correct phase since this is necessary to obtain the effect aimed at.

In Figure 'l no special amplifier tube is provided for the low frequency potential to be superposed, and this low frequency potential is simply derived from the same transformer T1 from which the potential applied to the modulation stage M is likewise taken. For the purpose of setting the proper voltage, special potentiometers are provided.

In Figure 8, the low frequency potential to be superposed on the endstage L is derived from the transformer T1 whose primary is placed in the plateV circuit of the modulation stage M. As a matter of fact, also the modulation frequency proper is present in the plate circuit of a modulation stage. If the tube L would be followed by a further tube, in like manner a low frequency transformer would be placed in the plate circuit of tube L. lIf the low frequency potential instead of being superposed in the tube L would only have to be superposed in the subsequent stage, then the secondary of the transformer T1 could be placed at the grid circuit of this subsequent tube. Such embodiments of the invention can be easily carried out in View of the above explanations.

The invention having thus been described, what is claimed is:

`1. A system for transmitting modulated carrier waves comprising, a source of carrier waves, a modulating tube and a power amplifier tube, each of said tubes having controlling electrodes, said modulating tube also having an output electrode, means for feeding waves from said source to a controlling electrode of said modulating tube, means for supplying modulating waves to said modulating tube, means for supplying modulated waves from the output electrode of said modulating tube to a controlling electrode of said power amplifier tube, means for biasing a controlling electrode of said power amplifier tube negative relative to the-cathode of said power amplifier tube, and means for simultaneously varying the potential of a controlling electrode of said power amplifier tube in accordance with said modulating waves and substantially in phase with the envelope of said modulated waves, said bias and Varying potential being such as to cause said power amplifier to operate entirely on the linear portion of its characteristic curve and at the'lower vend thereof to thereby reduce power amplifier therein the carrier wave energy, means for sup-- plying modulated waves from the anode of said modulating tube to the control grid of said power Y amplifier, means for simultaneously supplying modulating waves from the anode of said modulating tube to the vcontrol grid of said power amplifier tube, said last modulating Vwaves being substantially in phase with the envelope of said modulated wave, and means for biasing the control grid of said poweramplifier tube relative to the cathode of said power amplifier tube 'by a potential such that the instantaneous potential, on said control grid of said power amplifier tube causes the same to operate entirely on the linear portion of its characteristic and at the lower end thereof irrespective of variations in amplitude of said modulated Waves to thereby reduce power tube losses.

3. The method of increasing the economy of operation of transmitters by superimposing the modulated radio frequency wave and the modulation frequencies in an amplier tube of said transmitter, which tube has a control grid and a cathode which includes the steps of, biasing the control grid of said amplifier tube to a negative potential relative to the cathode such that said tube operates on the lower linear portion of its characteristic for waveamplitudes of the order of the carrier wave amplitude, impressing said modulated radio frequency wave on said control grid and impressing the modulating frequencies on said control grid in phase with the envelope of the modulated radio frequency wave to shift said point of operation up and down said characteristic curve when the envelope of said modulated wave exceeds in amplitude or is less than said carrier amplitude whereby the tube l'oss is maintained minimum throughout the modulation envelope cycle.

HELMUT PITSCH. 

